Ferroelectrics are very attractive substances because of their peculiar characteristics (such as very high relative permittivity, and good pyroelectric, piezoelectric and ferroelectric properties). So, various devices (such as ceramic multilayer capacitors, pyroelectric devices, piezoelectric devices and ferroelectric memories) have been developed and put into use utilizing such peculiar properties. Typical ferroelectrics are perovskite materials such as barium titanate (BaTiO3) and lead zirconate titanate (Pb(Zr1-xTix)O3, PZT). Of these, lead zirconate titanates (PZTs) provide relatively excellent polarization and piezoelectric properties and are therefore most widely used.
Lead-containing PZTs are specified hazardous substances. However, because there are currently no suitable commercially available alternative pyroelectric or piezoelectric materials, PZTs are exempt from the RoHS directive (the directive on the restriction of the use of specified hazardous substances in electrical and electronic equipment enforced by the European Union and Council of Europe). However, with the growing worldwide responsibility towards global environment conservation, a strong demand exists for development of pyroelectric and piezoelectric devices using lead-free ferroelectric materials.
Also, with the recent trend toward smaller and lighter electronic devices, there is an increasing need for ferroelectric thin-film devices in which a thin-film technology is utilized.
Herein, pyroelectric and piezoelectric thin-film devices will be described below as representatives of such ferroelectric thin-film devices. Piezoelectric devices utilize the piezoelectric effect of a ferroelectric material, and are widely used as functional devices such as actuators and stress sensors. Actuators generate a displacement or vibration in response to an applied voltage to a ferroelectric (piezoelectric) material. Stress sensors generate a voltage in response to a strain produced in a piezoelectric material. Pyroelectric devices detect light (including infrared light) utilizing the pyroelectric effect of a ferroelectric material, and are widely used as infrared human body sensors, etc.
Examples of piezoelectric devices utilizing a lead-free piezoelectric material are described below. Patent Literature 1 discloses a piezoelectric thin-film device including, on a substrate, a lower electrode, a piezoelectric thin-film and an upper electrode. The piezoelectric thin-film is made of an alkali niobate-based perovskite dielectric material of a chemical formula (NaxKyLiz)NbO3 (where 0<x<1, 0<y<1, 0≤z<1, and x+y+z=1). A buffer layer of a perovskite crystal structure material is formed between the piezoelectric thin-film and the lower electrode. The perovskite buffer layer is highly preferentially (001), (100), (010) or (111) oriented. According to this Patent Literature 1, the piezoelectric thin-film device utilizing the lead-free lithium potassium sodium niobate thin-film exhibits sufficient piezoelectric properties.
The practical use of the lead-free piezoelectric thin-film device requires reliability (durability) in long-term use in addition to improvement in piezoelectric properties. Various techniques intended for that have been developed.
For example, Patent Literature 2 discloses a piezoelectric film element including: a substrate; a lower electrode structure provided on the substrate; a KNN piezoelectric film provided on the lower electrode structure; at least one diffusion prevention structure provided on the KNN piezoelectric film; and an upper electrode structure provided on the diffusion prevention structure. The upper electrode structure includes an upper electrode made of an Au layer. The diffusion prevention structure includes, in order from the KNN piezoelectric film, an intermediate adhesive layer and an Au diffusion prevention layer that is provided on the intermediate adhesive layer and prevents diffusion of Au from the upper electrode to the KNN piezoelectric film.
Patent Literature 2 also discloses that the Au diffusion prevention layer is made of a Pt layer or a Pt alloy layer and that the intermediate adhesive layer is made of a Cr layer, a Ta layer, a Ti layer, a Ti oxide layer, or a Ti nitride layer. According to Patent Literature 2, a practical piezoelectric film device in which the electric insulation of the KNN piezoelectric film hardly deteriorates in long-term use and a piezoelectric film apparatus including the piezoelectric film device can be provided.